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"Green Rust" Shows Promise In Eliminating Selenium Soil Contamination

Date:

January 28, 1998

Source:

Lawrence Berkeley National Laboratory

Summary:

A surprising alternative to microorganisms for immobilizing selenium contamination in soil and sediment has been identified by researchers at the U.S. Department of Energy's Lawrence Berkeley National Laboratory. Green rust, a harmless natural iron oxide, was shown to chemically react with toxic selenium, converting it to a safer elemental form.

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BERKELEY, CA. -- A surprising alternative to microorganisms for immobilizing selenium contamination in soil and sediment has been identified by researchers at the U.S. Department of Energy's Lawrence Berkeley National Laboratory. Green rust, a harmless natural iron oxide, was shown to chemically react with toxic selenium, converting it to a safer elemental form.

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Selenium is a trace mineral that can be highly toxic or carcinogenic to humans and wildlife. The poisoning deaths of wild birds at the Kesterson Reservoir in the San Joaquin Valley in the early 1980s have been attributed to selenium in drainage from irrigation water. The incident was a graphic demonstration of how agricultural development can result in the accumulation of abnormally high and potentially lethal concentrations of selenium and other trace contaminants in soils and sediments.

Selenium's fate in contaminated soils has long been linked to the decomposition of plant material and other microbial activity, which was thought to be the primary means by which soluble, chemically active forms of selenium could be reduced to an elemental state. Elemental selenium is insoluble, which means it is less of a threat to move up through the soil into the food chain, or down through the soil into the groundwater.

Contrary to this past belief, however, a laboratory study led by Satish Myneni of Berkeley Lab's Earth Sciences Division, has revealed that green rust has the same effect as microorganisms on soluble forms of selenium.

"We have shown that the selenium transformation reaction in sediments and soils reduction can take place without the presence of the bacteria, via a different mechanism," says Myneni.

Joining him in this study were Tetsu Tokunaga, also with Berkeley Lab's Earth Sciences Division, and Gordon Brown, Jr., at the Stanford Synchrotron Radiation Laboratory. Their results were reported in a recent issue of the journal SCIENCE (11/7/97).

Although Myneni and his colleagues are not proposing any remediation strategy for selenium contaminated sites based on green rust, future cleanups and environmental management efforts depend upon a thorough understanding of selenium's basic chemistry and geochemical cycling. Furthermore, the green rust transformation reactions they have identified in selenium should also apply to other trace contaminants as well, such as chromium, and chlorinated hydrocarbons.

The researchers analyzed their reactions using various x-ray beam techniques, including x-ray absorption near edge structure (XANES), and extended x-ray absorption fine structure spectroscopy (EXAFS). A key to their findings was that the selenium transformation reactions take place under conditions of oxygen-depletion such as in the sediment beneath ponded water. These are the same conditions under which green rust is formed.

"Other researchers have shown that elemental iron and ferrous oxides can reduce soluble selenium to a less active state, but, unfortunately, these two forms of iron oxides do not occur in nature," says Myneni. "On the other hand, recent thermodynamic and kinetics studies show that green rust may be an important mineral in anoxic sediments."

Myneni and his colleagues are now in the process of analyzing samples of soils and sediments collected from selenium-contaminated sites for the presence of green rust. For this work, they will use the x-ray microscopy beamline at Berkeley Lab's Advanced Light Source.

The Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research and is managed by the University of California.

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